Universal ph indicator



Patented June 23, 1953 2,643,230 UNIVERSAL pH INDICATOR Shannon Mooradian, Somerville, and Henry E. Millson, Plainfield, N. J assignors to American Gyanamid Company, New York, N. Y., a corporation of Maine No Drawing. Application September 12, 1950, Serial No. 184,516

1 Claim. 1

This invention relates to a universal indicator solution capable of determining the pH of leather, cloth, and paper in the range of pH .1 to pH 9., and to a process for the preparation of this solution. More specifically, this invention relates to a universal indicator solution which is free from organic solvents and which does not undergo chromatographic separation when applied to textiles.

The dyeing and finishing of leather and certain textiles, particularly textiles containing basic nitrogenous fibers, are chemical operations, in which pH conditions are frequently critical. In these operations the materials being processed may be regarded as reagents, and therefore it becomes important to'determine initially the degree of acidity or alkalinity these materials possess.

For example, a piece of a woolen fabric in an acid condition generally dyes quite unevenly when certain milling or direct dyes are applied thereto, whereas the same fabric in a neutral or slightly alkaline condition generally dyes perfectly level. As a result, a demand has arisen for an inexpensive, rapid and portable means for determining the pH of leather and textiles within the range pH 1 to pH 9.

This demand has been augmented by developments in the paper art. It has become a frequent necessity to determine the pH of paper, particularly where it is foreseeable that the paper may come in contact with corrosive metals such as steel, or with the human skin.

Probably the most accurate way of determining the pH of leather, paper and textiles is to extract these materials with distilled water and to determine the pH of the extract by means of the Beckmann pH meter. Such a procedure, however, is undesirably slow and requires costly apparatus and trained personnel. Since the dyeing and finishing operations referred to above require that the pH values of the materials thus processed be determined routinely and rapidly by. untrained personnel, it has been proposed to utilize conventional laboratory indicator solutions for this purpose. These solutions consist of a number 'of dyes dissolved in an organic liquid, each of the dyes changing color at a different pH value so that the solutions exhibit a continuous spectrum of colors in passing through the particular pH range in which they are operative.

The above-mentioned proposal involved placing a drop of one of these indicator solutions on the surface of the fabric. It was expected that the drop would yield a stain having one definite color, and that by comparing the color of the stain thus formed with the color and its pH value given on an accompanying chart, the pH of the fabric could be determined in the same manner as had been done in the case of solutions. When the indicators of the prior art were applied in this manner, however, it was frequently impossible to make an accurate determination of the color .of the stain, as the drop yielded, particularly on wet fabrics, not a solidly colored stain, but a stain having a central portion of one color, a surrounding zone of a distinctly different color, and, not unusually, a third zone of a still different color. Under these circumstances it was impossible for the operator to determine with certainty the true pH of the fabric.

The discovery has now been made that the development of the above-noted colored zones is a chromatographic phenomenon, which arises from the fact that the universal indicator solutions suitable for employment in the paper, leather, and textile trades contain an organic liquid as a portion of the solvent medium. When a drop of such a solution is allowed to fall on a wet fabric, the dyes contained therein are selectively adsorbed by the fibers in accordance with the general principles of chromatogoraphy, so that the dye least strongly adsorbed diffuses to the boundary of the drop, impoverishing the central zone of dye, and distorted color values thus result throughout the whole stain.

It has further been discovered that universal pH indicators which are free from organic liquids do not undergo such chromatographic separation when applied to paper, leather, and textiles. It has finally been discovered that a concentrated organic liquid-free, universal pH indicator solution can be prepared as set forth below by an intensive mulling or grinding procedure from dyes ordinarily thought to be water-insoluble which is distinctly responsive to pH changes in the broad range of pH 1 to 9, and, which does not undergo the same chromatographic separation.

It is an advantage of the present invention that the indicator solutions may be prepared from readily available dyes and chemicals and that no special equipment is necessary.

According to the process of the present invention, there first is prepared a hot, very dilute and slightly alkaline solution of bromothymol blue in water and a similar solution is prepared from meta-cresol purple. Solubilization of the dyes is effected by intensively mulling the dyes with hot dilute alkali metal hydroxide solution. The proportion of water in these solutions is at least such that at room temperature no precipitation of the dye takes place, and preferably is in excess thereof. The weight of the blue should be about twice the Weight of the purple. The solutions are allowed to cool to room temperature, and are then combined, care being taken that no undissolved dye is present. To this mixed dye solution is added a concentrated, and preferably a hot saturated solution of rosolic acid in water, the weight of the rosolic acid being about of the weight of the meta-cresol purple. The resulting solution (hereinafter called the stock solution) is diluted with pure water (to form a laboratory solution) until the concentration of the dye therein is about This value is not critical, and merely affords a solution yielding a stain of generally acceptable intensity The pH solution of the instant invention is characterized by its freedom from noticeable odor.

Neither the pH of the stock nor the pH of the laboratory solution is important, but we prefer to adjust it to about 7.0 or 7.3 for the sake of uniformity.

Our preferred procedure for preparing both the stock and laboratory solutions is as follows, it bein understood that the example is illustrative of the method and composition, and is not in limitation of the invention which has been set forth above.

EXAMPLE 1. Bromothymol blue solution.100 ml. of N/lO sodium hydroxide diluted to 200 ml. with 100 ml. of distilled water having a pH of about 5.8 is heated to 180 F. and 5.44 g. of bromothymol blue (powder) is weighed out. About 500 mg. of this powder is very thoroughly milled in a mortar with about mg. of the hot sodium hydroxide solution until the rate of solution becomes relatively very slow. The resulting dye solution is poured from the undissolved dye remaining in the mortar, and the mulling is repeated, fresh powder and fresh, hot sodium hydroxide solution being added as required. If the resulting solution is not blue, it is made so by adding the minimum quantity of the sodium hydroxide solution. Hot distilled water is added until the volume of the solution is 2670 ml. The pH is then adjusted to about 7.5.

2. Meta-cresol purple solution-An aqueous solution of meta-cresol purple is prepared in the same manner using, however, 2.57 g. of metacresol purple powder and about 180 ml. of N /l0 sodium hydroxide solution. When solution of the dye is complete, hot distilled water (180 F.) is added to make up the solution to a volume of 3080 ml. and the pH is adjusted to about 7.5.

3. Mixing.The above solutions are cooled to room temperature over twelve hours, and are mixed carefully in a fresh vessel to prevent any undissolved dye which may be present from becoming a part of the mixture.

4. Rosolic acid.100 mg. of rosolic acid powder is slowly added to 600 ml. of hot (180 F.)-

distilled water, and the resulting mixture is brought to the boil with stirring. The solution is immediately filtered hot.

5. Stock solution.500 ml. of the hot rosolic acid filtrate containing about 0.084 g. of rosolic acid is added to the combined bromothymol blue-meta-cresol purple solutions. The product is about 6250 ml. of a stable stock solution.

6. Laboratory solution.13,700 ml. of cold distilled water is add-ed to the resulting stock solution and the pH of the product is adjusted to about 7 to 7.3. About 20,000 m1. of laboratory solution is thus obtained. It will be observed that the laboratory solution thus prepared contains about .0272% bromothymol blue, about .0128% meta-cresol purple, and about .0004% rosolic acid.

The steps recited in the example above are illustrative only and may be replaced by any other procedure which will yield the desired solutions free from undissolved particles of dye. For example, the sodium hydroxide may be replaced by known equivalents such as potassium hydrox- 4 ide. Distilled water need not be employed and water which has been purified of its anion and cation content by means of ion-exchange resins may also be used. Moreover, solution may be effected by dissolving the dyes in a water-soluble solvent therefor, heating the resulting mixture, slowly adding a large quantity of boiling water, and thereafter stripping oil the solvent. This stripping operation is a very difficult and slow one and we prefer to use the procedure set forth in the example.

The dye solutions of the present invention pass through a series of clearly defined color changes within the range pH 1 to pH 9 and each solution may be calibrated by known methods. The colors exhibited by the solution, the preparation of which has been set forth' in the example above, is as follows:

Color Red.

Yellowish orange.

Yellow. Yellowish green. Green.

Blue. Rcddish blue.

This nomenclature is in accordance with the terminology used by the dye colorist.

The indicator solution of the instant invention is applied with observation of the following precautions.

When heavy fabrics are tested the material should be cut and one or two drops of the indicator solution applied to the freshly cut edge. F'or light-weight fabrics, sk-eins, top rawstock, and paper, one or two drops of the indicator solution should be applied to the surface. The correct pH is indicated when the solution has penetrated the fibers.

The pH of colorless solutions may be tested by placing a few drops of the solution in question on a spot plate and adding two drops of the indicator solution. For colored solutions, i. e. dye baths, the original color of the bath must be taken into account when evaluating the resultant color change.

All readings should be made within one minute after the indicator solution has been applied.

Test paper may be impregnated with the indicator solution of the present invention by means well-known in the art, to form pH test papers which exhibits color changes characteristic of the solutions from which they were derived. Moreover, a wetting agent such as 0.01% of sodium di-Z-ethylhexylsulfosuccinate, may be added to the indicator solution. These proposals, however, play no part in the instant invention.

We claim:

As a new composition of matter, a universal pH indicator for determining the pH of protein fibers within the pH range of 1-9 comprising a water solution containing about .0272% bromothymol blue, about .0128% meta-cresol purple, and about .0004% rosolic acid.

SHANNON MOORADIAN. HENRY E. MILLSON.

References Cited in the file of this patent Acid-Base Indicators, by I. M. Kolthoif, pp. 128, 130, 162 and -177, The MacMillan Co., New York, 1937.

Handbook of Chemistry, N. A. Lange, 4th Ed, 1941, p. 466-7. 

